There have been conventionally known brushless motors in which a coil is placed on the stator side and a rotor having magnetic poles is rotated and thus a brush in contact with the rotor is eliminated. In such a brushless motor, it is required to estimate the position where the rotor is at a stop (hereafter, referred to as “rotor position”) when the motor is started. However, it is difficult to estimate rotor position unlike in motors having a brush.
The simplest method for estimating the rotor position of a brushless motor is to detect it with a sensor. In this case, however, increase in the number of parts and other like problems result. To cope with this, there have been conventionally proposed methods of supplying voltage to a coil installed in a stator and estimating the rotor position from change in the current passed through the coil. Refer to Patent Document 1, for example.
In the technology disclosed in the above patent document, rotor position is estimated by: supplying voltage so that magnetic flux in the direction perpendicular to the direction of rotation of a rotor is produced at predetermined angular intervals; and measuring the magnitude of wiring current that is the current passed through a coil. More specific description will be given. When so-called magnetic saturation, in which change in magnetic flux density is reduced even though current is increased, occurs in a coil, the inductance of the coil under a certain voltage is reduced. For this reason, a phenomenon of increase in current occurs. At this time, magnetic saturation is prone to occur by the magnetic flux produced by the rotor itself at an angle corresponding to the rotor position. In conventional technologies, therefore, rotor position is estimated by: supplying voltage more than once so that the direction of magnetic flux is changed at predetermined angular intervals; and detecting the wiring current in each case and comparing the peak of the wiring current.    [Patent Document 1] JP-A-S63-69489 (U.S. Pat. No. 4,876,491)
It will be assumed that a brushless motor is used for a vehicle fuel pump. In this case, voltage supplied to a coil is from a vehicle battery and thus there is a possibility of fluctuation in voltage. For example, voltage drop may be caused by vehicle electric equipment or voltage rise may be caused by a battery externally connected at the time of failure or the like.
In the technology disclosed in Patent Document 1, the coil may not be magnetically saturated at a low voltage condition when a significant voltage drop is generated, since a voltage supplying period for generating magnetic flux is maintained constant. In the case that the coil is not saturated magnetically, degradation in the accuracy of rotor position estimation may be occurred by using a method comparing peak values of winding current, since a current increase becomes smaller.
On the other hand, in a high voltage condition when a voltage rise is caused, current value is increased as well as voltage value. For this reason, it is necessary to make circuit elements such as the coils and the switches performing the voltage supply capable of withstanding large current. Consequently, the system may become bigger.